This invention relates to sensor systems and to security alarms which incorporate sensor systems.
In British Patent Specification No. 2 306 035, to which reference should be made, there is described a support system for a shelf or similar surface on which one or more articles can be placed, the system including one or more piezoelectric devices for partially or wholly supporting the weight of the shelf or similar surface, and means whereby an output voltage will be generated proportional to the change of weight when objects are placed on or taken from the shelf or similar surface.
It is an object of the present invention to provide a sensor system which can be used to detect the attempted theft or damage of items displayed or stored by hanging them on walls, partitions or similar substantially vertical structures, as opposed to items which are displayed by placing them on shelves or similar substantially horizontal structures.
It is another object of the present invention to provide an improved sensor system which can be used to detect the attempted theft of, or tampering with, small and lightweight items displayed or stored on horizontal or inclined surfaces.
It is a further object of the present invention to provide a sensor system which includes improved means for filtering out the effects of environmental disturbances, such as vibration and wind-induced movement.
According to a first aspect of the present invention there is provided a sensor system for use in detecting movement of an item suspended from a wall or other substantially vertical structure, or for detecting movement of an item displayed or stored on a horizontal or inclined surface, the sensor system including a piezoceramic element which is mounted on a plate which, in use, is located between the item and the structure or surface, and means for detecting the generation of an output by the piezoceramic element as a result of deflection of the plate in either of two opposite directions as a result of movement of the item.
The plate may have a first edge portion which is attached to a mounting and is bent out of the plane of the remainder of the plate, the plate being positioned so that a second edge portion, diametrically opposite to the first edge portion, is in contact with the item, for example, adjacent the lower edge of an item in the case of an item suspended from a wall or other substantially vertical structure.
The arrangement is thus such that, in use, the weight of a painting or other item suspended from the wall or other substantially vertical structure will create a small degree of deflection of the plate. Pressing the painting or other item towards the structure will alter the degree of deflection and thus generate an output, while moving the painting or other item away from the structure will also alter the degree of deflection and generate an output.
The same effect will be obtained if an attempt is made to move an item displayed or stored on a horizontal or inclined surface.
The plate may alternatively be in the form of a brass or other metal disc which is attached at its one edge to a mounting and has its diametrically opposite edge in engagement with a pad fixed to or in contact with the item, with the arrangement such that the disc is caused to deflect and an output is generated by the piezoceramic element in response to movement of the item in either direction.
In a further alternative arrangement, the disc is mounted on a resiliently deformable metal strip or plate, for example, a spring steel strip, such that a part of the strip, typically an end of the strip, is in contact with the item, with the arrangement such that the strip is caused to deflect (and will transmit such deflection to the disc) in response to movement of the item in either direction and an output will be generated by the piezoceramic element.
According to a second aspect of the present invention there is provided a sensor system for detecting movement of an item suspended from a wall or other substantially vertical structure, the sensor system including a piezoceramic element fixed to one side of a plate and a permanent magnet fixed to the other side of the plate.
The plate is preferably in the form of a brass or other metal disc that is mounted within a housing in such manner that the periphery, or a portion of the periphery, of the disc is clamped while the centre portion of the disc is free to move.
The piezoceramic element is preferably in a pre-stressed state when the sensor system is fitted to the item being protected.
According to a third aspect of the present invention there is provided a security alarm system which includes a sensor system as defined above for generating an output, a processing circuit for processing the output from the sensor system, and an alarm, the processing circuit including a slew rate limited anti-vibration low pass filter for differentiating between true alarm and false alarm signals.
Any suitable form of alarm may be employed. For example, it may be an audible siren, a radio or infra red transmitter, or a relay which changes the state of a switch in an external circuit.
The slew rate limited anti-vibration low pass filter is preferably arranged to filter out signals which have a duration less than a predetermined length of time and is preferably arranged in series with a pulse stretcher to ensure that the length of time for which the alarm is operated is greater than the length of time for which an output is generated by the slew rate limited anti-vibration low pass filter.
The arrangement may be such that, for example, signals having a duration of less than one second are filtered out by the slew rate limited anti-vibration low pass filter. Normal filters are amplitude dependent. The slew rate limited anti-vibration low pass filter is, however, not amplitude dependent.
The alarm may form part of a unit which, in the case of a wall-hung item, is located between the item and the wall. Alternatively, the alarm may be at a remote location with a signal transmitted to the alarm by means of an induction loop device.